Protein phosphorylation-dephosphorylation represents one of the most powerful and versatile mechanisms of molecular regulation in living organisms. For many years, however, protein phosphorylation-dephosphorylation was considered to be exclusive to eukaryotes. It was only after a long period of controversy that the existence of this modification was documented in bacteria (for a review, see [1]). Early studies essentially focused on the characterization of the “two-component system” [2] and “phosphotransferase PTS system” [3], the well-known hallmarks of bacterial signalling and regulation in which proteins are phosphorylated on histidines and aspartic acids. Then, thanks in large part to genomics the widespread presence of genes encoding eukaryotic-like serine/threonine kinases [4] and phosphatases has also turned out to be indisputable in bacteria [5]. In addition, high accuracy mass spectrometry experiments have recently allowed the characterization of more than one hundred of serine and threonine phosphorylation sites in two model bacteria [6,7].
Besides, some bacterial members of the large family of P-loop containing proteins [8,9] characterized by the Walker A nucleotide binding motif [10] were found to carry a protein kinase activity [11]. The first structurally [12] and functionally [13] characterized member of this new family of P-loop containing protein kinases was a serine kinase, the bifunctional HPr kinase/phosphorylase involved in a signalling pathway regulating the use of carbon sources by bacteria [14].
Progress on tyrosine phosphorylation in bacteria was slower and it was only in 1997 that the first gene encoding a bacterial tyrosine kinase was characterized [15]. This enzyme is structurally and functionally unrelated to its eukaryotic counterparts. Like the HPr kinase/phosphorylase, it belongs to the family of P-loop containing protein kinases. This particular type of tyrosine kinases has been identified in numerous bacteria [16] thus defining a bacterial idiosyncratic family of BY-kinases (for Bacterial tYrosine kinases) [17].
In proteobacteria and actinobacteria, BY-kinases are encoded as a single polypeptide whereas in firmicutes they are found in the form of two interacting proteins. The periplasmic N-terminal and the cytoplasmic C-terminal domains of BY-kinases from proteobacteria and actinobacteria are homologous to the membrane adaptator and the cytoplasmic BY-kinase from firmicutes, respectively. All BY-kinases that have been examined undergo autophosphorylation on a C-terminal tyrosine cluster, but also phosphorylate other proteins. Among the first identified endogenous protein substrates of BY-kinases were proteins involved in polysaccharide production [18-20] but also RNA polymerase sigma factors [21] and single-stranded DNA binding proteins [22]. Therefore, BY-kinases are implicated in many other important physiological processes including stress response, DNA metabolism, antibiotic resistance, control of the bacterial cell cycle, and pathogenicity [17,23].
The mechanism by which BY-kinases control extracellular polysaccharide biosynthesis is the best documented. Since 2000, an increasing number of publications have analyzed this process and tyrosine phosphorylation has turned out to be a key feature of capsule formation [24]. BY-kinases have been characterized as polysaccharide co-polymerases (PCP) belonging to multiprotein transmembrane machineries involved in synthesis and/or export of a large number of extracellular polysaccharides [25]. However, the accurate function of their phosphorylation remains unclear even if it has been shown to influence both the length and the amount of the produced polymer [26.-28], thus modifying the physico-chemical properties of the capsule [29]. Capsule play critical roles in the virulence of pathogenic bacteria. For instance, in bacterial human pathogens such as Staphylococcus aureus, capsule promotes virulence in animal models of infection [30,31]. The S. aureus capsule has been shown to be involved in protection against phagocytosis [32] and in modulation of the host immune response [33]. Therefore, bacterial tyrosine kinases can thus be considered as potential therapeutic targets.